In drilling, and in particular deep drilling (such as that required in the mining and oil industries), it is not usually possible, or sometimes desirable, to drill down to the required area in a straight line. This may be due to the nature of the material through which drilling is taking place (e.g. a particularly hard or unstable section of rock), or due to the probable location of the target area that the drilling is attempting to reach. It is therefore desirable to be able to control the direction of drilling of the drill bit, in order to steer the drill string and drill assembly toward a target zone within the material (usually a rock formation) being drilled.
It is known to re-direct the drill bit by installing specially designed equipment, which cannot be used for vertical drilling. One method uses a bent housing tool to allow the drill bit to initiate a progression face at a required angle (Kravits, et al., “Directional Drilling Techniques for Exploration In-advance of Mining”, World Mining Equipment, 1994). The disadvantage of these techniques is that they are time consuming, requiring the fitting and then un-fitting of the special tools whenever a change in direction is required. Moreover, the direction change is limited, based upon the geometries of the available tools, and fine control is not possible.
Other techniques have been developed to try and solve these problems. A more elaborate solution involves employing a steerable “mud motor” which may be installed in the drill assembly and controlled remotely from the surface (“How Does Directional Drilling Work?”, rigzone.com, [http://www.]rigzone.com/training/insight.asp?insight_id=295&c_id=1, accessed 14 May 2012). Typically, a mud motor is capable of channeling fluid preferentially to one side of the drill assembly, creating a force perpendicular to the axis of drilling, which “pushes” the drill assembly in the desired direction. This has the advantage that it does not involve fitting and un-fitting of tools, and also allows more fine control. However, there is still a need to increase the accuracy of the control, and there is a further disadvantage that the drill assembly is made highly complex by the addition of further machinery, which adds to its mass, and, as with any additional complexity, can be the cause of costly malfunction.